1. Field of the Invention
The present invention relates to a cellulose acylate film, a manufacturing method of a cellulose acylate film by stretching, an optically compensatory sheet, a polarizing plate and a liquid crystal display device, specifically relates to a cellulose acylate film having excellent optical characteristics by the specification of a crystallization index and a modulus of elasticity of the in-plane of a film, a manufacturing method of a cellulose acylate film by stretching having excellent optical characteristics by the specification of a crystallization index before stretching, a cellulose acylate film manufactured by the manufacturing method, an optically compensatory sheet containing the cellulose acylate film, and a polarizing plate and a liquid crystal display device using the optically compensatory sheet.
2. Description of the Related Art
Cellulose acylate films have been widely used as the protective film of a polarizing plate for a liquid crystal display device for the reasons of having proper water permeability and high optical isotropy (a retardation value is low).
In recent years, a method of giving phase difference to a cellulose acylate film to provide an optical compensation function in addition to the function as the protective film of a polarizing plate is proposed. For example, a method of adding a triazine type compound high in flatness to cellulose acylate to thereby reveal retardation is disclosed in European Patent Application No. 911656. However, with the increase of the requirement for high grade display devices mainly for liquid crystal television use, higher precise retardation control is also required of optically compensatory films.
As one problem of the control of retardation of a cellulose acylate film, the control of the ratio of in-plane retardation Re and retardation in the thickness direction Rth is exemplified. In-plane retardation Re is generally given to a cellulose acylate film by stretching by means of a tenter that is high in productivity. However, according to this method, not only Re but also Rth increase, so that a desired Re/Rth ratio cannot be obtained. In contrast to this, a manufacturing method of a cellulose acylate film having characteristics of retardation of high Re/Rth ratio by controlling the residual amount of solvent at the time of beginning stretching to a specific range is disclosed in JP-A-2003-170492 (the term “JP-A” as used herein refers to an “unexamined published Japanese patent application”). However, although the Re/Rth ratio becomes high according to this method, revealing of Re is insufficient to make a display device in high contrast and high grade, and there is a problem that the contrast variation tends to occur. Therefore, the improvement has been required.